Ok. This is going to be a tough one. I admit I didn't really read your first post. I usually read to the point where there is engine noise and automatically go to the RCA type and how they were run. I apologize for not reading your entire post.

The first thing we need to do is isolate the problem. This may require you to make a few trips to Radio Shack to buy some stuff. None of it will be very expensive though :thumbup:

Here's what to do:

First, does the thumping sound occur with the *edit - engine* off?

Isolate the HU from the rest of the system to determine if the problem is in the HU. To do this, you need to use a different source. A discman works well here. If you have a discman or can get one from a friend, use this. In order to get the discman to hook up to your amp, you'll need to buy one of these.

http://www.radioshack.com/product.as...5Fid=42%2D2483

This will hook up the discman to the amps. Make sure that you hook the cable up to the discman LINE OUT not the Headphone jack. If your discman does not have a LINE OUT, the you will need to use the headphone jack. Just don't turn the volume up very far.

If the noise is still there, then the problem does not reside in the HU. It is somewhere else. The next thing to try would be to test the RCA's. Use these

http://www.radioshack.com/product.as...Fid=274%2D1548

To plug your RCA cables to the discman then to the amp. If the problem comes back, the problem is the RCA cables.

If the problem doesn't come back, then you have a problem with one of the amps.

Try to take one of the amps out completely to see if that fixes the problem. If it doesn't, then you may need to replace the amp(s).

Good luck.

Mish